Cleaner forms of storing energy are in great demand. Examples of clean energy storage include rechargeable lithium (Li) ion batteries (i.e., Li-secondary batteries), in which Li+ ions move from the negative electrode to the positive electrode during discharge. In numerous applications (e.g., portable electronics and transportation), it is advantageous to use a solid-state Li ion battery which consists of primarily all solid-state materials as opposed to one that includes liquid components (e.g., flammable liquid electrolytes which include organic solvents such as alkylene carbonates), due to safety as well as energy density considerations. Solid-state Li ion batteries, which incorporate a Li-metal negative electrode, advantageously, have significantly lower electrode volumes and correspondingly increased energy densities.
Components of a solid-state battery include the solid-state electrolyte, which electrically isolates the positive and negative electrodes, and, often, also a catholyte, which is mixed with a positive electrode active material to improve the ionic conductivity in the space between positive electrode active material particles within the positive electrode region. Limitations in solid-state electrolytes have been a factor in preventing the commercialization of solid-state batteries. A third component, in some Li ion solid-state batteries, is an anolyte, which is laminated to, or in contact with, a negative electrode material (e.g., Li-metal). Many currently available electrolyte, catholyte, and anolyte materials, however, may not be stable within solid-state battery operating voltage ranges or when in contact with certain cathode (e.g., metal fluorides) or anode active materials (e.g., Li-metal).
Li-stuffed garnet is a class of oxides that has the potential to be suitable for use as a catholyte, electrolyte, and/or, anolyte in a solid-state battery. Certain garnet materials and processing techniques are known (e.g., U.S. Pat. Nos. 8,658,317; 8,092,941; and 7,901,658; US Patent Application Publication Nos. 2013/0085055, 2011/0281175, 2014/0093785, and 2014/0170504; also Bonderer, et al. “Free-Standing Ultrathin Ceramic Foils,” Journal of the American Ceramic Society, 2010, 93(11):3624-3631; and Murugan, et al., Angew Chem. Int. Ed. 2007, 46, 7778-7781), but these materials and techniques suffer from deficiencies which must be overcome for solid-state batteries to be commercially viable.
The state of the art teaches that lithium-stuffed garnet-based electrolytes, when used for Li ion rechargeable batteries, should be phase pure-cubic Li7La3Zr2O12, only, or cubic Li7La3Zr2O12 doped with the minimal amount of Al and/or Al2O3 that will not form secondary crystalline phases or inclusions in the primary cubic Li7La3Zr2O12 phase. The state of the art teaches that to prepare a lithium-stuffed garnet-based electrolyte with the highest Li+ ionic conductivity it is important to make the garnet phase pure—having only a single type of crystalline phase present. For example, the state of the art teaches that it is important to keep the amount of Al and/or Al2O3 below their solubility limit in Li7La3Zr2O12 in order not to precipitate insoluble secondary crystalline phases. See, for example, Matsuda, et. al., RSC Adv., 2016, 6, 78210, which sets forth that cubic phase garnet structures have a higher ionic conductivity than tetragonal phase garnet structures and which also sets forth certain compositions, e.g., a tetragonal phase aluminum doped garnet, Li7−xAlyLa3Zr2−xTaxO12, which remains tetragonal when x+3y<0.4 and which transforms to a cubic garnet when the empirical formula is Li6.6−z/2Alz/20.4La3Zr1.6+zTa0.4−zO12.
Certain garnets, which don't include lithium, are known to have a certain amount of secondary phase content therein (e.g., U.S. Pat. No. 8,461,535; U.S. Patent Application Publication No. 2016/0362341).
Lithium-stuffed garnet has the empirical formula Li7La3Zr2O12 (and is referred to in the art as “LLZO” or “LLZ”). This composition can exist in a variety of crystalline phases. For example, this composition is stable in a tetragonal phase at room temperature and this tetragonal phase has a low lithium-conductivity. This composition also forms a cubic phase, which has a much higher conductivity than the tetragonal phase. The cubic phase is formed by doping LLZO with aliovalent dopants such as aluminum (Al), niobium (Nb), tantalum (Ta) and similar dopants. Another example of LLZO is Li7−3xAlxLa3Zr2O12, wherein x is a rational number greater than zero and less than or equal to 0.2. In Li7−3xAlxLa3Zr2O12, the solubility limit of aluminum (Al) in the LLZO lattice is near 0.2. This means that if more than 0.2 moles of Al per LLZO mole are present, that additional amount of Al will precipitate out as a secondary phase (e.g., LaAlO3, LiAlO2, and La2Zr2O7). The state of the art teaches that LLZO should not be doped with Al beyond this solubility limit because these secondary phases will precipitate. For example, see Kotobuki, et. al., Journal of Power Sources 196 (2011) 7750-7754, which teaches that La2Zr2O7 impurities (a type of secondary phase) should be avoided during the formation of LLZO in order to produce a phase pure LLZO-based electrolyte which has a high Li ion conductivity.
Further improvements in garnet-based electrolytes are needed in order to commercialize solid-state batteries. Set forth herein are such improvements in addition to other disclosures.